Among many fields where superconductive wires are practically used is a high resolution nuclear magnetic resonance (NMR) spectroscopy using a superconductive magnet. As resolution of the spectroscopy increases by a higher magnetic field, superconductive magnets excellent in a high magnetic field characteristic are being preferably used more and more.
For the superconductive wire used in the superconductive magnet for generating a high magnetic field, Nb3Sn wire is put into practical use. The manufacture of Nb3Sn superconductive wires is usually done by employing a bronze method. According to the bronze method, plural Nb based wires are buried in a Cu—Sn based alloy (bronze) matrix and a drawing process is performed thereon to make the Nb based wire filaments. This multifilamentary superconductive wire group is then buried in copper for stabilization (stabilized copper) and undergoes a drawing process. Later, the drawn wire group is subjected to a heat treatment (diffusion heat treatment) at a temperature between 600° C. and 800° C. and as a result, an Nb3Sn compound is produced on the interface between the Nb based filament and the matrix (for example, refer to Non-Patent Literature 1). However, there is a limit to the concentration of Sn that can be dissolved in bronze (about 15.8 mass % or below), which leads to a thin Nb3Sn layer and deterioration in both crystallinity and high magnetic field characteristic.
A powder technique is another known method besides the bronze method to manufacture Nb3Sn superconductive wires. A typical example of the powder technique is ECN method, in which an intermediate compound powder of Nb and Sn is filled as a core material into an Nb sheath (tube-shaped body) and is subjected to a heat treatment to produce an Nb3Sn layer on the interface between the core material and the Nb sheath.
Unlike the bronze method, the ECN method does not set any limit to the concentration of Sn that can be dissolved, and is capable of producing a considerably thick Nb3Sn layer, thereby improving the characteristic of superconductivity. In addition, the ECN method is known to provide a very high critical current density per unit wire cross-sectional area since a non-superconductive area portion can be reduced as much as possible while a superconductive area portion is increased (refer to Non-Patent Literature 2).
Still according to another example of the powder technique (fusion-diffusion powder technique), Ta and Sn are subjected to a melt-diffusion reaction and a product thereof is pulverized to obtain a Ta—Sn alloy powder. The powder is filled into a sheath material made of Nb or an Nb based alloy as a core material (hereinafter referred to as ‘powder core part’), and diameter-reduction process and heat treatment are performed thereon (refer to Patent Literature 1). This method also does not have any limit to the quantity of Sn to be used, and the interactive diffusion between Ta and Nb forms an Nb3Sn layer thicker than the Nb3Sn layers by the conventional bronze method and ECN method. Consequently, a superconductive wire excellent in a high magnetic field characteristic is obtained.
FIG. 1 is a cross sectional view diagrammatically showing a manufacture of an Nb3Sn superconductive wire using a powder technique. In FIG. 1, reference numeral 1 denotes a sheath (tube-shaped body) made of Nb or an Nb based alloy, and reference numeral 2 denotes a powder core part filled with raw powder. To carry out the powder technique, the raw powder containing Sn is filled into the powder core part 2 of the sheath 1, and a diameter-reduction process, e.g., a drawing process, is performed when it is pushed out. The wire thus formed is then wound around a magnet and heat-treated to form an Nb3Sn superconductive layer on the interface between the sheath and the raw powder.
Examples of the raw powder in this case include Ta powder, a mixture of Nb powder and Sn powder, and an intermetallic compound powder obtained from reaction by heat treating Nb powder and Sn powder. In case of the intermetallic compound powder, a ball mill or a jet mill is utilized to pulverize the material after the reaction.
For Ta powder used in the ECN method or the melt-diffusion method, hydrogen may be added to make the Ta powder harden and this hardened Ta powder is mechanically pulverized (hereinafter referred to as “Ta powder with added H”) or dissolved by electron beams (hereinafter referred to as “EB powder”). Meanwhile, Sn powder is generally obtained by water or air atomization process.
Although the heat treatment temperature for the formation of superconductive layer is 930° C. or above, it can be lowered down to 750° C. by adding Cu to the raw powder. From this light, ECN or melt-diffusion method adds a very small amount of Cu powder to the raw powder before carrying out heat treatment for producing an intermetallic compound, or disposes a thin Cu layer on the inner side of the sheath. In addition, even though FIG. 1 illustrated a single filament wire, in practice, a lot of multifilamentary wires are placed in a Cu matrix.
The conventional powder techniques mentioned so far pose the following problems in raw powder. First, when Ta powder with added H or EB powder is used, a sintered body is hardened substantially after heat treatment for producing an intermetallic compound (hereinafter referred to as “MD heat treatment”). Thus, pulverization process becomes a very hard task and the operation time overall is much extended. Especially when the Sn concentration is greater than 50 atom %, the pulverization of Ta—Sn powder becomes very difficult. Although pulverization may be possible, the diameter of compound powder is so great that the sheath may be damaged during the drawing process. This also affects the superconductivity property to a considerable extent. In the worst case, the sheath may be broken and the manufacture of superconductive wires itself becomes difficult.
The particle shape of Ta powder with added H is shown in FIG. 2 (electron microscope picture), and the particle shape of Ta powder dissolved by EB is shown in FIG. 3 (electron microscope picture), respectively.
In addition, when the powder contains too much oxygen gas or hydrogen gas, workability or reactivity is deteriorated, hydrogen may be released during MD heat treatment which is dangerous, and degree of vacuum is not increased so one has to wait until the gas is completely escaped from the powder.
The surface of Sn powder is easily oxidized. If an oxide exists on the surface, reactivity is substantially deteriorated during MD heat treatment. Further, the resulting powder structure after the heat treatment is non-uniform, only degrading the wire characteristic.
Although it is typical to mix Cu powder with the raw powder for use in the manufacture of a superconductive wire, if the Cu powder is added prior to the MD heat treatment, relatively large Cu—Sn compounds are produced. Such compounds are very hard and brittle, so it becomes difficult to make wires of a uniform structure.
Moreover, according to the examination by the present inventors, if a Cu—Sn compound is present during the heat treatment for producing an Nb3Sn superconductive layer, a void is created after the reaction and the uniformity of the wire is not achieved. In addition, they observed another problem that if a Cu—Sn compound exists, Sn or a Sn alloy is easily effused from the end portions during the heat treatment for producing an Nb3Sn superconductive layer.
In order to fill (or pack) the raw powder into a sheath, a one-axis press is usually employed. However, a pressure pf 10 MPa at the most is applied in such filling method and the actual filling rate of the powder is only about 50%. When the wire process proceeded in that status nevertheless, a uniform structure in the longitudinal direction cannot be obtained and thus, part of the sheath may be damaged.
[Non-Patent Literature 1] K. Tachikawa Filamentary A15 Superconductors, Plenum Press (1980) p1
[Non-Patent Literature 2] W. L. Neijmeijer et al., J. Less-common Metal, Vol. 160 (1990) p161
[Patent Literature 1] Japanese Patent Laid-Open No. H11-250749